On-board processing used a dedicated 6MHz Z80B processor. Associated with this was an 8k operating system EPROM, 2k workspace RAM and 8k video space RAM. All 256 possible characters could be redefined in software. The video system also used a HD46505 CRTC chip. Hardware features allowed for the board’s output to be blanked or inverted. Also blinking, half-tone background and half-intensity character attributes were available.

An interesting feature was the capability of connecting the system keyboard (serial or parallel type) via this card. This provided a type-ahead buffer and programmable function keys (on the Gemini keyboards). Also fitted was a “beeper” which could be controlled in software. A light-pen socket was fitted and supported in the control program – returning character cell co-ordinates when triggered.

One of the interesting points about this card is that, like it’s earlier version the GM812, communication with the system processor was via three I/O ports. This meant that the 64k address space of the system processor was not taken up by video memory. The flipside of this was that some operations took longer – scrolling being noticeable due to screen updating not taking place during screen memory access by the CRTC. Due to inbuilt keyboard buffering on the video card this did not cause a real problem as the system could keep up with fast typists!

There was 1kB spare RAM available in the workspace area and it was quite possible to write short programs to run in there – independent of the main processor. Just a bit of early multi-tasking there…